Network connected devices have been the object of considerable interest and effort in recent times. Improvements in wireless communication techniques are leading to the creation of an “Internet of Things”.
Several wireless communication protocols have been proposed in this context. The LoRa communication system, known among others by patent applications EP2767847 and EP2449690, uses Chirp spread-spectrum modulation to achieve long transmission ranges with low power consumption and complexity.
It is also known to use diversity techniques to extract information from a noisy communication channel. Antenna diversity, Site diversity, and Frequency diversity, for example, foresees the use of several physical antennas, location or frequency bands to receive the same message. Several demodulation techniques have been devised to reconstruct a message from corrupted signals.
Wireless communication in IoT system are often deployed over license-free ISM bands. This means that message exchanges must contend with interfering signals from other devices using the same frequencies at the same time.
A same system may need to transmit over different interfering channels. The LoRa modulation system, for example allows simultaneous transmission at different spreading factors. Overlapping signals with different spreading factors can be independently modulated, but they do interfere with one another. Some variants of the LoRa modulation system also foresee transmission of FSK modulated data at the same time as chirp-modulated signals, and these signals can interfere as well.
Conventionally, transmission errors are dealt with by causing the retransmission of the corrupted message or part of it. In Automatic Repeat Request (ARQ) systems, the receiver verifies whether a received frame has been received correctly by using some embedded error-correction data, for example a CRC code. If the verification fails, a retransmission request is sent back to the transmitter. In other cases, the receiver would send an acknowledgement for any correctly received frame, missing which the frame will be retransmitted automatically.
The disadvantage of these schemes is that they represent an overhead on the network capacity, and the volume of exchanged data increases rapidly with the number of interferences. In some cases, the rate of error due to interference may approach 50%. In such situations, retransmission requests and repetition of previous frames may consume most of the network capacity. The requests, acknowledgments, and retransmissions may also be felt as interferences by other networks competing for the same frequencies, which will cause an increase of signalling in those networks as well, and may lead to an escalation of interference.
More advanced schemes, loosely termed “Hybrid ARQ” foresee the retransmission of a subset of the lost frame, or the retransmission of a different version of redundancy. Such measures also require active signal from the receiver and, while they mitigate the above limitations, cannot solve them completely.
In IoT network, the above strategy may prove inadequate because usually the duty-cycle of the base station is limited. Therefore, the preferred way to handle frame errors is to apply redundancy systematically: each frame is sent several times, possibly over different frequencies. This has of course a negative impact on battery life, network capacity, and interference into other networks.